1. Field of the Invention
The present invention relates to switched-capacitor circuits and more specifically to switched-capacitor circuits of very high resolution.
2. Discussion of the Related Art
A switched-capacitor circuit comprises at least one capacitor having at least one armature alternately connected, at a switching frequency, to one or the other of two terminals by switches. In operation, the assembly formed by the capacitor and the associated switches has an equivalent resistance equal to the ratio of the switching period to the capacitance of the capacitor. Switched-capacitor circuits have many advantages. They enable to simulate a variable resistance which depends on the switching frequency. This is the reason why switched-capacitor circuits are especially used for the forming of filters having a cut-off frequency depending on the switching frequency. Further, for equivalent manufacturing technologies, a switched-capacitor circuit takes up a smaller surface area when integrated than that which would be taken up by an equivalent circuit formed with real resistors. Further, for equivalent manufacturing technologies, the capacitance of an integrated capacitor may be obtained with an accuracy greater than that of a resistor.
The current tendency is to form switched-capacitor circuits of very high resolution, that is, for which the signal-to-noise ratio or SNR is greater than some hundred decibels. For a conventional switched-capacitor circuit, this requires increasing the capacitances of the circuit capacitors up to values that can exceed several hundreds of picofarads. The capacitance increase of the capacitors translates as an unwanted increase of the total consumption of the switched-capacitor circuit. It is further generally difficult to integrate capacitors with high capacitances.